Where Cassini ended mission, go beyond it (6)
I believe the answer is:
saturn
'where cassini ended mission' is the definition.
(the Cassini space probe was sent to Saturn)
'go beyond it' is the wordplay.
'go' becomes 'turn' (as in one's go or turn in a game).
'beyond' is a charade indicator (letters next to each other).
'it' becomes 'sa' (SA can mean sex appeal, as in 'it').
'turn' put after 'sa' is 'SATURN'.
(Other definitions for saturn that I've seen before include "Massive body ringed" , "Runs at (anag) - Roman god" , "tried to pass vessel" , "Sixth rock from the sun" , "Giant planet" .)